Guanosine-3′,5′-cyclic monophosphate (cGMP) is an intracellular signaling molecule that transmits signals such as nitric oxide (NO) and natriuretic peptides that enter cells coming from the outside thereof. Examples of receptor proteins of cGMP known to date include a cGMP-dependent protein phosphorylation enzyme (protein kinase G), a cGMP control channel, and a cGMP-degrading enzyme (phosphodiesterase, PDE). In particular, the initial reaction for signaling mediated by cGMP is thought to be activation of protein kinase G. Protein kinase G is a serine/threonine phosphorylation enzyme, and is activated at a cGMP concentration of around μM. Regarding vascular smooth muscle cells, it is thought that protein kinase G phosphorylates several portions including serine (Ser) at position 695 of MBS (myosin-binding subunit) that composes MLCP (myosin light chain phosphatase) so that vascular smooth muscle cells are relaxed. Furthermore, protein kinase G phosphorylates Ser 683 and Ser 696 (bovine type) of an inositol trisphosphate (IP3) receptor-associated cGKI substrate (IRAG) existing in the endoplasmic reticulum, thereby inhibiting the release of Ca (induced by IP3) from the endoplasmic reticulum. In addition to this, it is known that the Ca-dependent K channel, ATP-sensitive K channel, and L-, N-, and T-type channels are activated by phosphorylation. Phosphorylation of the Ca channel at the nerve endings is thought to promote the release of neurotransmitters. PDE5 activity is also regulated by protein kinase G such that cGMP is degraded to GMP by the PDE5 activated via phosphorylation of PDE5 at Ser92 (human type) induced by protein kinase G.
Hence, a drug that activates protein kinase G is useful as a therapeutic drug for hypertension, pulmonary hypertension, angina pectoris, arteriosclerotic cardiovascular diseases, erectile dysfunction, and the like. Examples of known compounds that activate protein kinase G include:
8-bromoguanosine-3′,5′-cyclic monophosphate (8-Br-cGMP), 8-bromoguanosine-3′,5′-cyclic monophosphorothioate, Sp-isomer (Sp-8-Br-cGMPS), 8-(4-chlorophenylthio)guanosine-3′,5′-cyclic monophosphate (8-pCPT-cGMP), 8-(4-chlorophenylthio)guanosine-3′,5′-cyclic monophosphate, acetoxymethyl ester (8-pCPT-cGMP-AM), 8-(4-chlorophenylthio)guanosine-3′,5′-cyclic monophosphorothioate, Sp-isomer (Sp-8-pCPT-cGMPS), 8-(4-chlorophenylthio)-β-phenyl-1,N2-ethenoguanosine-3′,5′-cyclic monophosphate (8-pCPT-PET-cGMP), 8-(4-chlorophenylthio)-β-phenyl-1,N2-ethenoguanosine-3′,5′-cyclic monophosphorothioate, Sp-isomer (Sp-8-pCPT-PET-cGMPS), guanosine-3′,5′-cyclic monophosphorothioate, Sp-isomer (Sp-cGMPS), 8-bromo-β-phenyl-1,N2-ethenoguanosine-3′,5′-cyclic monophosphorothioate, Sp-isomer (Sp-8-Br-PET-cGMPS), 1-aminoguanosine-3′,5′-cyclic monophosphate (1-NH2-cGMP), and 8-(2-aminophenylthio) guanosine-3′,5′-cyclic monophosphate (8-APT-cGMP). cGMP agonists are generally used as research reagents for the analysis of intracellular signaling mechanism mediated by cGMP. In particular, 8-Br-cGMP is frequently used and is known to have not only an in vitro effect in a culture cell system, but also in an in vivo vasodilator effect (Eur J Pharmacol. 118, 155-161 (1985)).
Moreover, organic nitrate preparations such as nitroglycerin which activate soluble guanylate cyclase, sildenafil which specifically inhibits PDE5 (which is a cGMP-degrading enzyme), and the like, also have an effect of elevating intracellular cGMP concentration. They all exert their pharmacological effects by indirectly activating protein kinase G and have already been clinically used as pharmaceutical products for treating hypertension, angina pectoris, erectile dysfunction, and the like.